1. Field of the Invention
This invention generally relates to a non-impact type printer and in particular to an ink-jet printer which discharges a series of charged ink droplets toward a recording medium through the gap between a pair of deflection plates to which an electric field is applied. More specifically, the present invention relates to a method and system for controlling charging of ink droplets in a charging type ink-jet printer.
2. Description of the Prior Art
Several types of ink-jet printers are well-known in the art. The charging type ink-jet printer includes a printing head from which a series of charged ink droplets are discharged toward a recording medium placed against the printing head through the gap between oppositely placed deflection plates to which an electric field is applied. In one type of the ink-jet printer, ink droplets are uniformly charged and passed through the gap between a pair of deflection plates to which an electric field varying in accordance with an image signal is applied. And, in another type of the ink-jet printer, ink droplets are varyingly charged dependent upon an image signal and the thus charged ink droplets are passed through the gap between a pair of deflection plates to which a constant electric field is applied.
In prior art charging type ink-jet printers, ink droplets to be used for printing are charged but those ink droplets which are not used for printing are not charged positively. The noncharged particles are not deflected when they pass through the gap between the deflection plates and thus they are collected by a gutter and prevented from landing on the recording medium as well known in the art. In this case, however, when a series of charged ink droplets are continuously produced by the printer head to fly toward the recording medium, the leading charged ink droplet tends to be pushed forward due to the electrostatic interactions with the following ink droplets which are likely charged. Under the condition, it has been often observed that the first droplet of a series of charged ink droplets to be used for printing catches up with the immediately preceding noncharged ink droplet which is not to be used for printing. This is disadvantageous because print error or distortion will result.
Stated more in detail with reference to FIGS. 1 and 2, the ink droplets indicated by S.sub.1 -S.sub.3 are those which are not positively charged and thus are not to be used for printing; on the other hand, the ink droplets indicated by K.sub.1 -K.sub.4 are those which are positively charged to be used for printing. In this particular case, the ink droplets to be used for printing are charged in the positive polarity. As shown in FIG. 1, in the case where a series of charged ink droplets K.sub.1 -K.sub.4 are preceded by another series of noncharged ink droplets S.sub.3 -S.sub.1, the first droplet K.sub.1 of the charged ink droplet series is pushed forward to catch up with the last droplet S.sub.1 of the preceding noncharged droplet series thereby forming a larger-sized integrated droplet, as shown in FIG. 2, before being deflected by the electric field formed between the deflection plates. Such merging of ink droplets is not advantageous because the resulting printed characters will be distorted and low in quality.